This invention relates to spectroscopy and more particularly to spectroanalytical systems.
The history of spectroscopy is sprinkled with expressions of the need to get more light onto the detector. To avoid losses of light which necessarily arise when diffractive rays are focused by means of lenses, the concave diffraction grating was invented by Henry Rowland. The concave diffraction grating combines the functions of optical imaging and diffraction into one optical element. Traditional mountings for the concave spherical grating mounting use a slit source oriented to maximize resolution. The spectral lines obtained with a concave grating show the same aberrations as images obtained with a concave mirror, primarily astigmatism. For applications in which the detector is very small or in which the detector is at a remote location and light must be conducted to the detector via an optical fiber, the problem encountered may be likened to the old one of trying to put a square peg into a round hole. Astigmatism spreads the rays out along lines perpendicular to the plane of the Rowland circle, the amount of astigmatism varying with position on the circle.